The invention relates to a method for manufacturing conically shaped metal screen bodies for centrifugals, especially for household juice centrifugals. The invention also relates to centrifugals made according to the present method.
It is known to equip continuously operating centrifugals with conical metal screen bodies. Screen bodies for industrial centrifugal machines mostly rest on supporting baskets, whereas in household centrifugals the screen body is a load-bearing component in addition to its screen function. Therefore, the strength and inherent stability of the screen bodies in household centrifugals are of particular importance. Screen bodies used in households are subject to uncontrollable and improper handling, and that is why their strength and stability, as well as their resistance to cleaning and scouring agents are essential features to be carefully taken into account in their manufacture. Household centrifugals operate at relatively high speeds. For this reason, a high resistance and sturdiness of the screen body is very important to avoid household accidents.
Screen bodies of the afore-mentioned type are mass produced in rather great numbers. Therefore, manufacturing costs must be kept down in spite of said high demands on usability.
Prior art methods for manufacturing screen bodies for household juice centrifugals are rather complicated and costly and the known screen bodies have a number of serious disadvantages.
Hertofore the screen foils or thin screen metal sheets are made mainly by electroforming also known as galvanoplastic deposition. In order that for said process and the resultant small screen foil thicknesses the necessary mechanical strength and welding stability may be ensured, metal materials of the hard nickel type are commonly used to make the screen.
According to a known manufacturing process, a circular-sector screen foil is made by electroforming. This foil is then formed into the three-dimensional conical shape by welding or soldering the longitudinal edges of the circular-sector screen foil to each other. Separately prefabricated flange-like parts are then welded or soldered to the upper and lower ends. These flange-like parts consist of a material which is very thick compared to the thickness of the screen foil and have a correspondingly high weight. Both soldering and welding heat up the screen foil. Such heating makes the material of the screen brittle to such an extent that many screen bodies break already after a short time of use. This material fatigue is even accelerated by the comparatively high weight of the flange-like part at the upper end of the screen body due to the high speeds, to which the screen body in continuously working household juice centrifugals is subjected. Such high speeds produce correspondingly high mass forces. The slightest non-symmetric mass distribution between the lower flange-like part and the upper flange-like part, which cannot be avoided even by maintaining strict manufacturing tolerances, continuously cause such mass forces. These forces must be transmitted from the upper flange-like body or part to the lower flange-like body through the material of the screen foil, which is comparatively thin, weak and thermally preweakened in addition by the soldering or welding. The well known so-called vibration or flutter ruptures are the result of said known manufacturing process.
In another known process an attempt was made to provide the screen body of conical shape with the flange-like parts by spray attaching same, at least at the upper end. In this way the thermal loading at the upper rim of the screen foil could be prevented, however, said method also did not yield satisfactory results because there is a limit to the strength of sprayable plastic material, whereby the resultant weight of the upper flange-like part became quite high which led to the mentioned unfavorable dynamic loads on the screen foil. Besides, plastics material is not an ideal material for this purpose, because it does not have the necessary surface hardness required to withstand the effects of load components processed in household juice centrifugals, e.g., pits of berries or the like. Therefore, molded plastic parts became quickly dulled and/or show scratches and surface cracks. Plastics material, in addition, is very sensitive to such surface conditions in terms of material strength.
Another known process described in German Patent Publication No. 2,153,901 did not eliminate the weight problem as such, but it did avoid the disadvantageous influences on the strength of the screen body. According to said known process the screen body is composed of three circular-sector screen foils, the edges of which have bent areas serving as anchoring means. By using the anchoring means in appropriate molds, these three screen foil pieces are sprayed with plastic in such a way that a basket-like plastic body is produced which encloses the edges of the screen foil pieces. During this process, plastic ribs are formed along the surface line or generatrix of the screen body. These ribs are attached in an integral manner to the flange-like parts at the upper and lower end of the screen body. This manufacturing process, of course, mechanically relieves the screen foil pieces to a great extent, and the basket-like plastic body is the load bearing element proper. The screen bodies made in this way, however, have a relatively high weight and are afflicted with the disadvantages resulting from the use of plastic materials in this particular environment.
For the sake of completeness, another known process should be mentioned, wherein the three-dimensional conical shape of the screen foil is made by electroforming or deposition. This process has the advantage that the longitudinal seam for producing the three-dimensional conical shape may be avoided. However, substantial problems are encountered when the matrix dies required for electroforming have to be regenerated. Therefore, this known process also does not solve the problem of economically manufacturing screen bodies.